﻿#define _CRT_SECURE_NO_WARNINGS
#include<iostream>
#include<cstring>
#include<algorithm>

using namespace std;
typedef long long LL;

int main()
{
	int n;
	scanf("%d",&n);

	LL sum = (n + 1ll) * n / 2;
	for (int i = 0; i < n - 1; i++)
	{
		int x;
		scanf("%d",&x);
		sum -= x;
	}
	printf("%lld\n", sum);
	return 0;
}